It is known that in automatic variable-speed gearboxes with planetary gearset systems, disk clutches and disk brakes are used for engaging the individual gears. These are actuated by pressure medium operated piston-cylinder arrangements whose pressure medium supply is controlled by a control and regulation unit.
Such transmission shifting elements, however, besides certain advantages, also have the disadvantage of causing comparatively high power loss even when not engaged because of the relative rotation between the steel disks and lining disks. This power loss can only be reduced by having as large a separation as possible between the steel and lining disks, which sums up to give the so-termed air clearance.
As a rule, this necessary air clearance is adjusted by a restoring spring that acts upon the shifting element in the opening direction of the actuation piston. However, this creates the disadvantage that owing to the force of the restoring spring the piston-cylinder arrangement has to be made larger, and when the actuation piston is acted upon by a clutch-closing pressure the actuation path corresponding to the air clearance first has to be covered in a pre-filling phase before the disks come into force-transferring engagement. In typical automatic gearboxes it takes approximately 200 ms to cover the air clearance path, and this is perceived as a long time particularly by drivers of sports vehicles. Especially in vehicles having automatic gearboxes with sequential shift systems, in which the transmission gears of the automatic gearbox can be engaged manually by means of a trip-switch, such shifting delays are complained of as slowness of the reaction time.
Accordingly in the prior art, it has been proposed to make use of disk clutches with shifting sleeves and synchronization devices known from manual countershaft transmissions, by means of which in the open condition the disk clutch can be disengaged cleanly and free from any drag torque from the rotating gearbox components with which it is connected in the engaged condition.
This also has the advantage that the actuation piston then needs no air clearance and a restoring spring can be omitted. Advantageously, the aforesaid dead time of up to 200 ms during a transmission ratio change process then no longer occurs.
The rotation speed difference between the rotating transmission component and the component of the transmission shifting element to be driven when the latter is engaged is first overcome by way of the synchronizing device, so that thereafter, by an axial displacement of a shifting sleeve, a positive-lock connection is formed between the components in such manner that the drive torque can be transferred via the shifting sleeve. Once the speed equalization has taken place and the shifting sleeve connection has been formed, the gear and load synchronization usual in gearshift processes takes place by pressurizing the clutch piston that acts on the clutch disks.
Against this background, a gearbox is known from DE 100 40 116 A1, in which a positive-lock clutch is provided between an inner disk carrier of a disk brake and a planetary support of a planetary gearset. This positive-lock clutch is opened when the disk brake is open to uncouple transmission components from the disk brake, in order to reduce disadvantageous drag torques.
If it becomes necessary to close the disk brake, the positive-lock clutch is closed without force by an axial piston so that the transmission components, in this case the planetary support and the transmission components connected thereto, are coupled to an inner disk carrier of the disk brake.
Then a disk pack of the disk brake is acted upon by the axial piston with an adjustable closing force so that by virtue of this pressurization of the axial piston the brake torque can be gently and continuously increased without any appreciable engagement jerk.
A disadvantage of this gearbox, however, is that the positive-lock clutch between the inner disk carrier of the disk brake and the planetary support of the planetary gearset can only be closed comfortably when the inner disk carrier and planetary support are rotating at the same speed.
In contrast, if there is a rotation speed difference between the inner disk carrier and the planetary support, the positive-lock clutch cannot be engaged or only so with high expenditure of force and at the same time high noise emission.
To minimize the force used and the noise emitted when the positive-lock clutch is closed, it is necessary for the inner disk carrier and the planetary support to be at rest, which distinctly limits the utility range of this system.
Furthermore from the not previously published DE 102 44 523 A1, a gearbox is known with which the above disadvantages are avoided. According to this prior art, it is proposed to equip a gearbox with an additional synchronization device by way of which, before closing, a clutch mechanism is synchronized in relation to rotation speed by positive locking. This makes it possible to close the clutch mechanism using little force and without disturbing noise emission.
Although the last-mentioned gearbox represents a clear advance compared with the prior art first mentioned, there is still a need for further improvement. Namely, with both actuation arrangements, it is disadvantageous that in both a closing and an opening process the actuation piston for the disk clutch has to move through a comparatively large regulating distance, which is noticeable as a somewhat longer time required to carry out a transmission ratio change process in the gearbox. This is mainly because the actuation piston first has to move through the synchronization path of the synchronizing device before, by further piston actuation, the disk pack can be compressed or released in the closing or opening direction, respectively.
Against this background the purpose of the present invention is, therefore, to propose an automatic gearbox of the type described, having at least one transmission shifting element formed as a transmission clutch or transmission brake with a synchronization device, which can be actuated more rapidly than those of the prior art.